It is well known in the mineral processing arts that ferric ion either as ferric sulphate (Fe.sub.2 (SO.sub.4).sub.3) or ferric chloride (FeCl.sub.3), etc. can be employed for the leaching of copper and other base metals from sulphide ores or concentrate in accordance with the following typical reactions (chalcocite is used as a typical example of copper/base metal sulphides): ##EQU1##
If ferric chloride is employed, hydrochloric acid will substitute sulphuric acid in reactions (a) and (b) resulting in reactions (c) and (d): ##EQU2##
The dissolved copper sulphate can then be recovered by extracting it into an organic solvent such as kerosene with the use of a suitable extractant. This is followed by back extracting the copper into a sulphuric acid solution. The resulting copper containing sulphuric acid solution can then be subjected to an electrowinning process to produce pure cathodic copper metal.
However, hitherto direct use of ferric sulphate for the leaching of copper and base metal sulphides has not been commercially and technically feasible due to:
(i) the high cost of ferric sulphate/ferric chloride that would be required for the dissolution of copper (the stoichiometric requirement is 6.30 tonnes of ferric sulphate per tonne of copper from chalcocite (CU.sub.2 S)) and PA1 (ii) problems due to the presence of an excessive quantity of iron (in either ferric or ferrous state) which would interfere with the solvent extraction/electrowinning process for the purification and production of cathodic copper metal. PA1 (i) using bacteria such as thiobacillus ferrioxidant for the oxidation of ferrous sulphate to ferric sulphate. In fact, without bacterial activities, economic heap leaching of copper sulphide bearing ores would not be feasible; PA1 (ii) using a pressure vessel (autoclave) at an elevated temperature to oxidise ferrous sulphate to ferric sulphate during which time direct oxidation of some sulphide also occurs to a certain extent. PA1 forcing the ferrous ion-containing solution through an in-line mixer under the influence of a controlled pressure differential between an inlet and an outlet of the mixer; and PA1 injecting oxygen or an oxygen-containing chemical reagent into the in-line mixer to facilitate oxidation of said ferrous ions (Fe.sup.2+) to form ferric ions (Fe.sup.3+). PA1 dissolution of an insoluble base metal compound or uranium into a soluble metal compound in a leach slurry or solution by chemical oxidation with ferric ions (Fe.sup.3+) so as to produce a byproduct ferrous ion (Fe.sup.2+); PA1 recovering the base metal or uranium by extracting the soluble metal compound and subjecting it to a suitable winning process; PA1 recirculating the leach slurry or solution with the byproduct ferrous ion through an in-line mixer; and, PA1 converting the ferrous ion back to ferric ion by oxidation, wherein said oxidation is facilitated by injecting oxygen or an oxygen-containing chemical reagent into the in-line mixer.
In order to make the process commercially and technically feasible, one must be able to effect high copper recovery with the use of a relatively small quantity of ferric sulphate in the leach solution. Once the ferric sulphate is consumed and converted to ferrous state, it must be oxidised back to the ferric state (in accordance with reaction (e)). ##EQU3##
However, direct sparging of oxygen containing gases into solution is uneconomic due to low solubility of oxygen in solution resulting in high wastage of oxygen and long residence time. This problem is exacerbated because of the fact that an elevated temperature (eg 80-100.degree. C.) is often required to achieve appreciable ferrous oxidation and copper sulphide dissolution reactions.
Various approaches have therefore been investigated including:
In each case the object is to re-oxidise the ferrous sulphate back to the ferric sulphate state in accordance with reaction (e) in order to reutilise the ferric ion for the copper leaching duty (reactions (a) and (b)) without the need for excessive supply of ferric sulphate into the leach slurry.
In such operations, oxygen essentially plays the role of an oxidising agent whilst the ferric ion acts as an electron carrier for the copper sulphide oxidation process.
However, a major disadvantage with bacterial oxidation of ferrous ion is that it is very slow and normally takes many days to perform the task. It has therefore been found only suitable for heap leaching practice when the leaching time can be as long as 12 months or more. On the other hand, although pressure oxidation requires shorter residence time (in the order of a few hours) it incurs a high capital investment and high operating costs.
Ferric ion is also useful for the leaching of uranium from uranium ores due to its high oxidative property. In general, uranium occurs in nature as oxides in different oxidation states: U.sup.6+, U.sup.5+, U.sup.4+ and U.sup.3+. Whilst U.sup.6+ is readily soluble in sulphuric acid, other forms of uranium minerals are either sparingly soluble or not soluble at all.
In order to solubilize the uranium bearing minerals, it is essential to oxidise them to the U.sup.6+ state with the use of an oxidising agent such as hydrogen peroxide, sodium peroxide, or ferric ion in accordance to reaction (g). Direct oxidation of uraninite by oxygen in aqueous medium is normally not feasible. EQU UO.sub.2 +2FE.sup.3+ .fwdarw.UO.sub.2.sup.2+ +2Fe.sup.2+ (g)
As in the case of copper leaching, the use of ferric ion has not been commercially feasible, even though uranium is more valuable than copper (currently around $40/kg of yellow cake), due to the lack of an economic process for production/regeneration of ferric ion in leach solutions.
Existing industry practices include the use of either hydrogen peroxide, nitric acid, sodium peroxide or sodium chlorate. All of these chemical reagents are expensive and apart from hydrogen peroxide, the by-products are generally environmental unfriendly and hazardous to handle. Substituting these chemical reagents with a less expensive reagent, such as oxygen, would provide a very attractive alternative processing route.